1. Field of the Invention:
The present invention relates to an image forming apparatus.
2. Description of the Prior Art:
Various image forming apparatus are known in the art. One such image forming apparatus is a color electronic copying machine including an endless belt positioned near a photosensitive body or drum and reciprocally movable for conveying a copy sheet thereon. In such a color electronic copying machine, a toner image formed on the photosensitive body by color separation is transferred in an image transfer section to the copy sheet upon forward movement of the endless belt. After all toner images in given colors have been transferred to the color sheet in superimposed relation, the toner images are fixed to the copy sheet to produce a colored copy (see, for example, Japanese Laid-Open Patent Publication No. 58-198062).
The toner images on the copy sheet are fixed thereto by an image fixing device which moves the copy sheet at a speed different from the speed of travel through the image transfer section in order to produce the amount of heat that is required to fuse the toner images. Generally, the speed at which the copy sheet moves through the image fixing device is lower than the speed of travel through the image transfer section.
Where the image fixing device comprises a heating roller and a pressing roller which are held in contact with each other at peripheral surfaces, the copy sheet delivered from the image transfer section, with the toner images thereon, tends to sag due to the different speeds upon arrival at the image fixing device. As a result, the toner images on the copy sheet are rubbed by the roller.
In producing a color copy, it is important that the toner images be successively transferred from the photosensitive body onto the copy sheet in exact registry. To meet such a requirement in a color copying machine in which a document support table for supporting a document to be copied is reciprocally movable, the document support table and the endless belt are moved in synchronism with each other.
Such synchronous operation will be described in greater detail with reference to FIGS. 7 and 8 of the accompanying drawings. A document support table 102 with a document 101 placed thereon and a conveyor belt 103 disposed below the document support table 102 are moved in synchronism with each other in opposite directions by a driving mechanism (not shown). Between the document support table 102 and the conveyor belt 103, which may be an endless belt, there are disposed an exposure device 104, a photosensitive belt 105 positioned beneath the exposure device 104 and movable in the direction of the arrow A, the photosensitive belt 105 having a confronting the exposure device 104 for exposure of an image of the document 101 through the exposure device 104, and an image developing device 106 disposed near the photosensitive belt 105 for developing an electrostatic latent image carried on the photosensitive belt 105 into a visible toner image. An image transfer device (not shown) is disposed in a region where the photosensitive belt 105 confronts the conveyor belt 103 for transferring the developed image from the photosensitive belt 105 onto an image transfer sheet S such as a copy sheet which has been delivered by the conveyor belt 103.
When the document support table 102 is moved in the direction of the arrow in FIG. 7, the photosensitive belt 105 is exposed to a document image via the exposure device 104 to form an electrostatic latent image thereon. The electrostatic latent image on the photosensitive belt 105 is developed into a visible toner image by the image developing device 106. The visible toner image is then transferred electrostatically by the image transfer device onto the image transfer sheet S that travels in the direction of the arrow B on the conveyor belt 103.
In order to bring the leading end a of the image on the photosensitive belt 105 into registry with the leading end of the image transfer sheet S in FIG. 7, it is necessary that the leading end of the image transfer sheet S on the conveyor belt 103 be initially spaced from the position where it will contact the photosensitive belt 105 by a distance equal to the distance 1 from the exposure position via the image developing position to the image transfer position. Therefore, the conveyor belt 103 needs an additional length. After the trailing end al of the document 101 on the document support table 102 has been exposed as shown in FIG. 8, it is also necessary to move the document support table 102 by the distance 1 that the trailing end of the image on the photosensitive belt 105 moves from the exposure position to the image transfer position. The document support table 102 is thus required to move an additional stroke.
The additional length of the conveyor belt 103 and the additional stroke of travel of the document support table 102 are effective to prevent images from being displaced out of registry in a color copying machine in which images are superimposed on an image transfer sheet. However, the conveyor belt 103 requires an increased length, and, in addition, in case the copying machine is used in a monochoromatic copy mode, the sizes of the conveyor belt and the document support table are large as compared with the size of a document to be copied, with the result that the copying machine is of a large size.